Method for continuous acetylation of wood elements

ABSTRACT

The present invention relates to a method for continuous acetylation of wood elements. The acetylation is conducted with an acetylation medium at a pressure of at least 1.5 barg in a substantially oxygen free environment. Alternatively, the method according to the invention comprises the steps of: (a) feeding wood elements in a substantially oxygen free environment to a continuous acetylation reactor, and (b) treating the wood elements with an acetylation medium in the continuous acetylation reactor under wood acetylation reaction conditions, at a pressure of at least 1.5 barg. The process according to the present invention allows to acetylate wood elements to a high acetyl content in a very efficient way, without compromising on the quality of the material. The acetylated wood elements can be used in the production of medium density fibreboards with superior qualities such as dimensional stability and durability.

FIELD OF THE INVENTION

The invention relates to the field of wood acetylation. Particularly,the present invention provides a method for continuous acetylation ofwood elements.

BACKGROUND OF THE INVENTION

It has been known in the art to chemically modify wood with the aim toimprove its service life and particularly to acetylate wood. Therebymaterials with improved material properties, e.g. dimensional stability,hardness, durability, are obtained.

In the art, it is known to use batch processes, i.e. non steady stateprocesses for the acetylation of wood particles. Such processes,however, often lead to poor product uniformity as the properties differsignificantly during the cycle of each batch and potentially from batchto batch. Since long term steady state conditions are not established,no two batches can ever be considered as identical.

Some references address the option to conduct wood acetylation in acontinuous process. EP 0746570 discloses a process for the acetylationof lignocellulosic materials (LM) comprising a first step wherein the LMis brought into intimate contact with an acetylating agent comprisingacetic anhydride as the major component at a temperature from 80° C. to140° C. and bringing the acetylated LM from the first step into contactwith a heated gas inert under the reaction conditions in a stripper at atemperature above 140° C. A comparable process is described in EP0650998.

In the acetylation of wood elements, particularly of wood chips, atechnical challenge in providing a suitable continuous process, is tocombine the advantages of a continuous process, with the desired resultof a sufficiently high degree and uniformity of acetylation.

It is notoriously difficult to obtain acetylation degrees in terms ofacetyl content higher than 17 wt. % for wood elements. It should benoted that the acetylation degree used in the present invention ismeasured as an acetyl content (AC) in contrast to weight percentage gain(WPG). In the art, acetyl contents higher than 17 wt. % are typicallyobtained via catalyzed processes. In catalyzed acetylation processes, acatalyst for the acetylation reaction is added to the wood. These arefrequently toxic organic substances, such as pyridine, but also sodiumbicarbonate, potassium acetate, and other salts, particularly acetatesalts are known. The invention aims at providing a non-catalyzedprocess, thus avoiding the drawback of having residues of additionalforeign substances (viz. the catalyst) remain in the wood.

The present invention aims to provide a method for the acetylation ofwood elements, which is fast and hence cost-effective over batchprocesses, and at the same time allows the obtaining of wood elementsconsistent in quality (high acetyl content, no discolorations of theelements). The acetylated wood elements should have a high quality, e.g.to be used in the production of medium density fibreboards possessingsuperior properties such as dimensional stability and durability.Particularly, the invention also aims to provide a continuousacetylation process that enables the production of all wood elementshaving a relatively high degree of acetylation.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention presents, in one aspect, a method for continuous acetylationof wood elements, wherein the acetylation is conducted with anacetylation medium at a pressure of at least 1.5 barg in a substantiallyoxygen free environment.

In another aspect, the invention presents a method for continuousacetylation of wood elements comprising the steps of:

(a) feeding wood elements in a substantially oxygen free environment toa continuous acetylation reactor,

(b) treating the wood elements with an acetylation medium in thecontinuous acetylation reactor under wood acetylation reactionconditions, at a pressure of at least 1.5 barg.

DETAILED DESCRIPTION OF THE INVENTION

The invention, in a broad sense, is based on the unexpected finding thatit is possible to obtain acetylation wood elements with a high acetylcontent and excellent wood quality in an efficient continuousacetylation process, for example using a residence time of less than onehour. This is achieved by the combined effect of the acetylationconditions used (particularly, high pressures), a low oxygen, preferablya substantially oxygen-free, environment and a continuous manner ofperforming the acetylation reaction.

Pressures higher than 0.5 barg are rarely used in the acetylation ofwood. A common belief is that such high pressures in combination withhigh temperatures could damage the wood and lead to a lower quality,e.g. change in colour and strength of the acetylated wood elements. Inthe present invention this is however obviated by performing theacetylation in a reduced oxygen environment, for example by removing theoxygen from the wood elements before the acetylation.

Preferably, the wood elements to be acetylated have a moisture contentof at most 6 wt. %, more preferably at most 4 wt. % or even at most 3wt. %. The morphology of the wood elements may be, but is not limitedto, wood chips, wood fibres, wood flower, wood strands, wood shavings,etc. In a preferred embodiment, the wood elements are wood chips. Thewood elements preferably belong to non-durable wood species such as softwoods, for example, coniferous trees, spruce, pine or fir or tonon-durable hardwoods, typically beech, birch, eucalyptus, poplar oralder. Preferably, the wood elements have a size of a height and widthfrom 0.1 cm to 3 cm, and a length of 1 to 7 cm, and are more preferablyhave dimensions of approximately 2 to 3 cm×1 to 2 cm×1 to 2 cm in size,ideally approximately 2.5 cm×1.5 cm×1.5 cm in size.

After chipping the trees, the green wood elements have a typical naturalmoisture content (MC) of >100 wt. % based on the dry weight. Themoisture content of the elements is preferably lowered to below 6 wt. %dry weight, before the elements are treated in the acetylation process.At higher moisture contents too much of the acetylation medium(typically containing acetic anhydride) will be consumed in a reactionwith water in the wood instead of with the hydroxyl groups in the wood,which makes the acetylation process less efficient. Another disadvantageof a too high MC is that the strong exothermic reaction of aceticanhydride with water in the wood can lead to local overheating in thewood and discoloration of the wood elements may occur. More preferably,the moisture content is less than 4 wt. %, ideally less than 3 wt. %.Lowering of the moisture content can be achieved by conventionalcontinuous or batch drying techniques. In a particular embodiment, woodelements are dried in a continuous process and the dried elements arestored in a (blanketed) silo. A batch of these dried elements may betaken in order to prepare for the next step.

The method according to the invention involves conducting theacetylation reaction in a substantially oxygen free environment,preferably conducting the acetylation reaction in a reactor having anoxygen concentration of less than 1 vol. %.

In preferred embodiments, the wood elements fed to the acetylationreaction are in a substantially oxygen free environment. Preferably, theoxygen content of the wood elements fed to the acetylation reactor isless than 1 vol. %, more preferably less than 0.8 vol. %, yet morepreferably less than 0.5 vol. %. The oxygen may be removed by creating avacuum, pressure cycling, purging with an inert gas (such as nitrogen)or with a combination of these. In practice, wood elements can be placedin a vessel, which is evacuated to reach a certain vacuum level.Suitable pressures for applying vacuum are 100 mbar or less, preferably70 mbar or less, more preferably 50 mbar or less. Typically, a vacuum of100 mbar absolute or less is sufficient to remove the oxygen from thewood elements to a sufficiently low level in order to subsequentlyrealize a low oxygen, and preferably a substantially oxygen-free,environment in the acetylation reactor. It is not sufficient to onlyremove oxygen from the gaseous environment of the wood elements aspotentially some oxygen can still be present between or within the woodelements. Preferably, substantially all the gaseous oxygen is removedfrom the wood elements.

In other embodiments, the substantially oxygen free environment isdefined as having an oxygen content in the acetylation reactor below 1vol. %, preferably below 0.8 vol. %, more preferably below 0.5 vol. % oreven below 0.4 vol. %. The oxygen content can suitably be measured onthe effluent from the reactor.

Without wishing to be bound by a particular theory, it is believed thatlow oxygen content during acetylation prevents wood from excessiveoxidation that takes place at high temperatures and high acetylationpressures used. Such oxidation could lead to darkened but also lessstrong wood fibers. In addition, low oxygen, and preferablysubstantially oxygen-free, environment contributes to a safe and stableworking of the acetylation medium during the acetylation reaction,wherein the acetylation medium is preferably recycled and re-used. Theacetylation reaction and the associated quality of the resultant woodproduct is particularly enhanced when the wood introduced to theacetylation reactor has both a low oxygen and a low moisture content.

The step of removal of oxygen is typically carried out batch-wise. In apreferred embodiment, the wood elements are vacuumed in a hopper andafter the vacuum step are brought under inert atmosphere, preferably, ofnitrogen. The wood elements can be further purged, optionally pressurepurged, with inert, preferably nitrogen, gas or with vapours of theacetylating medium.

If the step of oxygen removal is carried out batch-wise, it willtypically be necessary to arrange for a transition between the batchstep and the continuous acetylation. Care should be taken not tointroduce oxygen during such transition into the environment of the woodelements. The transition step is carried out under inert atmosphere,preferably nitrogen. The transition can be done by transferring a batchof wood elements into a hopper with a continuous discharging device(e.g. screw at the bottom of the hopper) that continuously feeds to theacetylation reactor. Preferably, the wood elements are continuouslydischarged from the screw at the bottom of the hopper to a rotary valveset-up. Such a rotary valve set-up may comprise one or multiply rotaryvalves, with a variety of purging options through or upstream/downstreamof any rotary valve. The rotary valve set-up can then be designed tocontrol the interface between inert nitrogen (upstream of the rotaryvalve set-up) and hydrocarbon containing (downstream of the rotary valveset-up) atmosphere. The separation between the upstream and downstreamatmospheres is maintained by setting appropriate pressure differences,and if necessary purge streams, over the rotary valve set-up. In thisway a continuous feed from the upstream, optionally batch, part of theprocess to the continuous acetylation step can be made, withoutintroducing oxygen into the reaction environment.

In one embodiment, oxygen is removed first by applying vacuum to thewood elements and subsequently applying inert atmosphere, preferably ofnitrogen, to the wood elements to replace the air inside and between theelements with the inert gas or gases.

In some embodiments, in addition to the low oxygen environment, it isalso preferred to have a low nitrogen environment during theacetylation. Particularly, it can be achieved by removing nitrogen fromthe wood elements before the acetylation step. Nitrogen (or other inertgas or gases) used to remove the oxygen from the elements, can then bereplaced, e.g., with the gaseous acetylation medium. An advantage ofthis embodiment is that the gaseous acetylation medium has a higherthermal capacity than nitrogen and therefore provides for a moreeffective heating of the wood elements during the acetylation. It ispreferred that the nitrogen content in the reactor is less than 1 vol.%, preferably less than 0.8 vol. %, more preferably less than 0.5 vol.%, as measured on the effluent from the reactor. The removal of nitrogencan be realized by applying vacuum.

In a preferred embodiment, the wood elements after the removal of oxygen(and optionally moisture and/or nitrogen) are brought under increasedpressure before the acetylation, preferably at least 1.5 barg.

In a following step, the wood elements are treated with an acetylationmedium in a continuous acetylation reactor under wood acetylationreaction conditions to obtain acetylated wood elements. It is anadvantage of the present invention that no pre-impregnation stepupstream of the reactor is necessary and the wood elements can bedirectly acetylated in the acetylation reactor, which leads to more astreamlined process. Preferably, substantially all of the acetylationmedium is supplied to the wood elements after they are at a pressure ofat least 1.5 barg. Substantially means here the majority of theacetylation medium introduced for acetylation, preferably at least 80wt. %, more preferably at least 90 wt. %, yet more preferable at least95 wt. %.

In some embodiments, the method comprises the steps of providing woodelements, removing oxygen from the wood elements, feeding the woodelements in a substantially oxygen free continuous acetylation reactor,treating the wood elements with an acetylation medium in the continuousacetylation reactor under wood acetylation reaction conditions at apressure of at least 1.5 barg. Preferably, substantially all of theacetylation medium is supplied to the wood elements after they are at apressure of at least 1.5 barg.

Under “wood acetylation reaction conditions” it is understood theconditions under which the wood undergoes acetylation. Preferably, theacetylation conditions comprise heating to a temperature in the ranges150-220° C. or 150-200° C., preferably 160-190° C., alternatively165-185° C. or 160-180° C., or even 170-200° C. Preferably, theresidence time in the acetylation reactor is under 1 hour, morepreferably in the range 10-50 min. Preferably, the pressure in theacetylation reactor is in the range 1.5-5 barg, more preferably 2-3.5barg. Preferably, the acetylation reaction is conducted substantially inthe absence of catalyst.

It has been found that when judiciously bringing, ideallypre-conditioned, wood elements fed to an acetylation reactor to thedesired temperature and optionally a certain degree of saturation of thewood elements for the acetylation reaction it is possible to maintaingood product properties with a much shorter minimum residence time inthe reactor achieved by substantially all the wood elements than hadpreviously been considered possible. In this specification“pre-conditioned” means being substantially oxygen-free (as previouslydefined) and optionally also being substantially free of one or more ofnitrogen, moisture and absorbed hydrocarbons.

Under “acetylation medium” it is understood a chemical compound or amixture of compounds that is able to acetylate hydroxyl groups in thewood. Preferably, the acetylation medium comprises acetic acid, aceticanhydride or a mixture thereof. More preferably, the acetylation mediumcomprises a mixture of acetic acid and acetic anhydride. Particularly,such mixture—on feeding into the acetylation reactor— preferablycomprises at least 50 wt. %, more preferably at least 85 wt. % of aceticanhydride.

An important feature of the process is that it is a continuous process.In one embodiment, order to minimise the residence time and residencetime distribution of a continuous acetylation reactor, preferably a plugflow or “first in first out” principle is employed. In a preferredembodiment the plug flow or “first in first out” principle is fulfilledby using a screw conveyor, preferably substantially filled with the woodelements. Due to the gentle rotation of the screw the wood elements areprevented from attrition. This results in good wood element qualityduring the process. In a preferred embodiment, a vertical cylinder ismounted on top of the entrance of the screw and by constantly having alevel in this vertical cylinder the (nearly) horizontal screw issubstantially filled, preferably as completely as possible, that is, atleast 80 vol. %, more preferably at least 90 vol. %, yet more preferablyat least 95 vol. % of the available volume in the screw. This isbeneficial as it prevents by-passing of the heating gas, which isco-current with the wood elements flow.

Another important feature of the present process is a short residencetime in the acetylation reactor. “Residence time” means here the timethat substantially all of the wood elements spend in the acetylationreactor, from the point where they enter the reactor to the point theacetylated elements leave the reactor. In this case substantially allwood elements is defined as at least 80 wt. %, preferably at least 90wt. %, more preferably at least 95 wt. %, most preferably at least 99wt. %.

The reactor may have different zones, e.g. a preparation zone, anacetylation reaction zone (A). The residence time in the reactor willthen be a sum of the residence times in different zones of the reactor.Preferably, the wood elements have a residence time of less than 1 hourin the acetylation reactor. More preferably, the residence time in theacetylation reactor is in the range 10-50 min.

In one embodiment, wood elements are transported through the reactor insubstantially plug flow, optionally using the screw, with a well-definedresidence time (typically 20 to 40 minutes, such as about 20 minutes, 25minutes, 30 minutes, 35 minutes or 40 minutes) and residence timedistribution (preferably, substantially all wood elements have a minimumresidence time). Alternatively, the reactor is designed such thatsubstantially all wood elements have a minimum residence time of atleast 5 to 60 minutes, preferably 10-50 minutes, typically 20-40minutes, such as about 20 minutes, 25 minutes, 30 minutes, 35 minutes or40 minutes. In this case substantially all wood elements have a minimumresidence time defined as at least 90 wt. %, preferably at least 95 wt.%, more preferably at least 97 wt. %, most preferably at least 99 wt. %of all wood elements have the required minimum residence time.

In a preferred embodiment, the wood elements having a temperature within+/−15° C. of the set-point temperature of the desired acetylationreaction are introduced into acetylation reaction zone (A) having apressure of at least 1.5 barg and a residence time of under 1 hour.Preferably, the wood elements have a minimum degree of absorption withthe acetylation medium, as defined below, before entering acetylationreaction zone (A), more preferably, the wood elements are saturated inthe acetylation medium.

Preferably, the majority of the (liquid and/or vapour) acetylationmedium is added to the reactor upstream of acetylation reaction zone(A). Alternatively or additionally, the majority of the acetylationmedium is supplied to the wood elements after they are at a pressure ofat least 1.5 barg. Majority means here at least 70 wt. %, morepreferably at least 80 wt. %, yet more preferably at least 90 wt. % ofthe total acetylation medium introduced into the acetylation reactor.Most preferably, substantially all of liquid and/or vapour acetylationmedium is introduced to the reactor upstream of acetylation reactionzone (A) and/or after the wood elements are at a pressure of at least1.5 barg. Preferably, the average residence time in acetylation reactionzone (A) is greater than 65%, alternatively greater than 75%, preferablygreater than 80%, most preferably greater than 85% of the residence timeof the acetylation reactor.

An important feature of the process is the heat management of the woodelements during the acetylation step. The heating is preferably done byusing acetylation medium in vapour form. Preferably, the temperature ofthe acetylation vapour on introduction into the reactor is within 10-35°C. of the desired temperature for the acetylation process, e.g. whichcan preferably be 160-190° C. The deviation from this desiredtemperature can lie within 10-35° C. but preferably no more than 30° C.above the set-point temperature for the acetylation reaction inquestion. The acetylation medium is preferably an acetic acid/aceticanhydride mix, which is preferably rich in acetic anhydride, such ascomprising >85 wt. %, preferably >90 wt. % acetic anhydride. Withoutwishing to be bound by theory, the inventors believe that thecondensation of this vapour mix onto the relatively cool wood elementsleads to a rapid and effective heating of the wood, which makesefficient use of the residence time of the acetylation reactor.

Another part of the acetylation medium is preferably supplied in aliquid form. The liquid acetylation medium is preferably an aceticacid/acetic anhydride mix, which is preferably rich in acetic anhydride,such as comprising >85 wt. %, preferably >90 wt. % acetic anhydride. Theliquid acetylation medium is preferably brought in direct contact withthe wood elements in the reactor, which can be done e.g. by spraying ofthe liquid on the wood elements. In one embodiment, the temperature ofthe acetylation liquid on introduction into the reactor is within 10-35°C. of the bubble point at acetylation reaction conditions, e.g. whichcan preferably be 160-190° C. and 1.5-5 barg. In a preferred embodimentthe temperature of the acetylation liquid on introduction into thereactor is between 0° C. and 50° C., ideally between 10° C. and 30° C.,or alternatively at ambient temperature. It has been found to beimportant that the wood elements are only heated to within 10-35° C. ofthe desired temperature for the acetylation process once the woodelements have achieved a preferred minimum absorption of acetylationmedium. While not absolutely necessary, it is advantageous if the woodelements have a minimum absorption of the acetylation medium beforebeing subjected to acetylation reaction conditions. Such minimumabsorption can for example be 10 wt. %, more preferably 20 wt. %.Particularly good results are achieved when the wood elements contain atleast 30 wt. %, more preferably 40-60 wt. % of the liquid acetylationmedium, based on the dry weight of the elements.

Without wishing to be bound by theory, the inventors believe that addingthe acetylation medium in the liquid form in direct contact with thewood ensures that there is sufficient amount of acetic anhydrideavailable for acetylation, to reach the desired acetyl content during arelatively short residence time in the reactor. It is also believed thatin some embodiments, having a minimum degree of absorption in the woodelements prior to attaining a temperature of greater than 125° C.,preferably greater than 135° C., ideally greater than 150° C. underacetylation conditions enables the acetylation reactor minimum residencetime to be reduced whilst still achieving good product properties.

It is preferred to contact the chips with the liquid acetylation mediumin a section of the reactor where the elements can be contacteduniformly, ideally a majority, preferably at least 70 wt. %, morepreferably at least 80 wt. %, most preferably at least 90 wt. % orsubstantially all of the liquid acetylation medium added to theacetylation reactor is introduced onto the wood elements within thefirst 35%, alternatively the first 25%, preferably within the first 20%,most preferably the first 15% of the residence time of the acetylationreactor.

When the wood elements have not been pre-impregnated with acetylationmedium upstream of the reactor, it is particularly important tointroduce the liquid onto the wood elements as soon as possible. It isequally important to avoid introducing the majority of the liquid ontothe wood elements in any sections of the reactor where mixing of theelements with the liquid is constrained by the reactor design. In aparticular embodiment, the wood elements are sprayed with the liquidbefore being introduced in any screw section of the reactor. In otherembodiments, however, it may be advantageous to introduce the liquid inthe screw section of the reactor.

As described above, in a particularly preferred embodiment, theacetylation medium is supplied to the reactor in both gaseous and liquidform. The weight ratio of the acetylation medium in the gaseous form tothe liquid form supplied to the acetylation reactor is preferably in therange 0.5/1 to 4/1, more preferably, in the range 0.7/1 to 3.5/1,alternatively in the ranges 2/1 to 4/1 or 0/1 to 2/1, most preferably inthe range 1/1 to 3/1.

The unreacted acetylation medium after the acetylation reaction ispreferably recycled and re-used in the process. This can be achieved bycondensing the gas at the exit part of the reactor and separating aceticanhydride by distillation. The gas can be condensed using cooling waterat a near atmospheric pressure, or using air cooling. The distillationis preferably done at atmospheric or sub-atmospheric pressure. Separatedacetic anhydride can then be recycled to the reactor and separatedacetic acid can be stored or used in another process, or converted toacetic anhydride by known methods.

In a following step, the reactor can be depressurized and preferably,the acetylated wood elements are discharged from the reactor. In thisstep, the pressure typically drops to about 0-0.1 barg. One of thefeatures of an embodiment of the present process is that, as a result ofthe combined use of a vapour and liquid acetylation medium, theacetylated elements, before being depressurised, contain at least 55 wt.% liquid on dry basis of the acetylated wood elements weight.Preferably, for discharging of the acetylated material from the reactora rotary valve set-up is used. Such a rotary valve set-up may compriseone or multiply rotary valves, with a variety of purging options throughor upstream/downstream of any rotary valve. The separation between theupstream and downstream atmospheres is maintained by setting appropriatepressure differences, and if necessary purge streams, over the rotaryvalve set-up.

Further, the wood elements are preferably dried. Conventional dryingtechniques can be used, for example blowing with a hot gas over the woodelements. Preferably, the acetylated elements are dried at a temperaturein the range 150-200° C., more preferably in the range 170-190° C.Preferably, near-ambient pressure is used during drying. Drying of theacetylated wood particles may be conveniently done, e.g. by hotnitrogen, carbon dioxide or flue gas. During drying acetic acid andunreacted acetic anhydride are removed, preferably to a level of below0.5 wt. %. Drying can be effected in multiple steps.

The dried wood elements can then be cooled down to room temperature.Conventional techniques can be used for that such as air cooling orwater cooling.

As a result of the above-described process, wood elements withacetylation levels of at least 17 and up to about 25 wt. % acetylcontent (AC) may readily be obtained, as measured by high-pressureliquid chromatography (HPLC). The HPLC is used to quantify the acetateion concentration resulting from the saponification of acetyl groups.Preferably, the acetyl content is at least 18 wt. %, more preferably atleast 20 wt. %, yet more preferably at least 22 wt. %.

It should be noted that in determining wood acetylation degrees, twodifferent approaches are taken in the field. One is based on WPG (WeightPercentage Gain). WPG compares a sample before and after acetylationtreatment, and as a result any substances added (and any residues stillpresent in the wood) increase the value. WPG is explained in thefollowing formula: WPG=(M_(increase)/M_(sample before reaction))×100%.Herein M stands for mass, andM_(increase)=M_(sample after reaction)−M_(sample before reaction)).

The other approach, is to actually measure the acetyl content (AC). Thisis given as AC=(M_(acetyls)/M_(sample after reaction))×100%. TypicallyHPLC (high-pressure liquid chromatography) can be used to quantify theacetate ion concentrations resulting from the saponification of acetylgroups from the wood. From this the overall mass of the added acetylgroups after acetylation can be taken as M_(acetyls).

The different results for WPG and AC can be explained with reference tothe following theoretical example: a sample of, e.g., 1 g of wood isacetylated and after the reaction has a mass of 1.25 g. Thus M_(acetyls)is 0.25 g. The resulting WPG is: (1.25−1.00)/1.00*100%=25%. Calculatedas acetyl content, AC is =(1.25−1.00)/1.25*100%=20%.

Hence, care should be taken not to directly compare degrees ofacetylation expressed in WPG with degrees of acetylation expressed inAC. In the present description AC values are elected to identify thedegree of acetylation.

An advantage of the process according to the present invention is thatit allows to produce acetylated wood elements in a very efficient way,without compromising on the quality of the material and the acetylcontent. The process of the invention does not need an impregnation stepupstream of the reactor and has a particularly short residence time.

Acetylated wood elements according to the present invention may usefullybe refined and converted to a board, such as medium density fibreboard,which will possess superior dimensional stability, durability, stabilityto ultra-violet light and thermal conductivity, compared with boardsderived from non-acetylated wood particles. Moreover, the wood elementsacetylated according to the present invention do not only have a high ACbut also excellent physical properties such as appearance (nodiscoloration), fibre quality, length and strength. This can beevaluated by the properties of the end product, such as medium densityfibreboards.

The following non-limiting example serves to illustrate the invention.In this example and throughout this specification, all percentages,parts and ratios are by weight unless indicated otherwise.

Example I

A mixture of wood chips with dimensions 2.5 cm×1.5 cm×1.5 cm consistingof 80% Sitka Spruce with 20% Loblolly Pine has been dried to a moisturecontent of 2.5%. These wood chips are subsequently put in a vacuumvessel and evacuated to a pressure of less than 50 mbara. After reachingthis level the vessel is pressurized to 2.5 barg with nitrogen. Thismixture is subsequently fed into the acetylation step. In thisacetylation step the wood chips are contacted and heated by theacetylation medium consisting of acetic anhydride/acetic acid mixturewith 90:10 wt:wt composition. The wood chips are contacted with liquidacetic anhydride/acetic acid mixture and heated with superheated aceticanhydride/acetic acid vapour—with the liquid spray to superheated vapourratio being 1:2.5 wt:wt. The wood chips/acetylation medium mixture isheated to a temperature of 170° C. at a pressure of 2.3 barg by means ofsuperheated acetic anhydride/acetic acid mixture. Oxygen content duringthis reaction is below 0.25 vol. %. After a residence time of 30 minutesat these conditions the pressure is released and the wood chips aretransferred to the drying step, which proceeds at 0 barg. The residencetime for this drying step is 70 minutes, where the chips are heated withnitrogen to a temperature of 170° C. This process resulted in acetylatedwood chips with an acetyl content (AC) of 22.5% and a residual acidcontent of 0.5 wt. %.

Example II (Comparative)

A mixture of wood chips with dimensions 2.5 cm×1.5 cm×1.5 cm consistingof 80% Sitka Spruce with 20% Loblolly Pine has been dried to a moisturecontent of 2.5%. These wood chips are pressurized to 2.5 barg withnitrogen, without an oxygen removal step. This mixture is subsequentlyfed into the acetylation step. In this acetylation step the wood chipsare contacted and heated by the acetylation medium consisting of aceticanhydride/acetic acid mixture with 90:10 wt:wt composition. The woodchips are contacted with liquid acetic anhydride/acetic acid mixture andheated with superheated acetic anhydride/acetic acid vapour—with theliquid spray to superheated vapour ratio being 1:2.5 wt:wt. The woodchips/acetylation medium mixture is heated to a temperature of 170° C.at a pressure of 2.3 barg by means of superheated aceticanhydride/acetic acid mixture. Oxygen content during this reactionreached values over 5 vol. %. After a residence time of 30 minutes atthese conditions the pressure is released and the wood chips aretransferred to the drying step, which proceeds at 0 barg. The residencetime for this drying step is 70 minutes, where the chips are heated withnitrogen to a temperature of 170° C. This process resulted in acetylatedwood chips with an acetyl content (AC) of 22.1% and a residual acidcontent of 0.6 wt. %. The chips were heavily darkened in contrast to thechips from Example I.

1. A method for continuous acetylation of wood elements, wherein theacetylation is conducted with an acetylation medium under woodacetylation reaction conditions at a pressure of at least 1.5 barg in asubstantially oxygen free environment.
 2. A method for continuousacetylation of wood elements comprising the steps of: (a) feeding woodelements in a substantially oxygen free environment to a continuousacetylation reactor, (b) treating the wood elements with an acetylationmedium in the continuous acetylation reactor under wood acetylationreaction conditions, at a pressure of at least 1.5 barg.
 3. The methodaccording to claim 1, wherein the oxygen content in the acetylationreactor is less than 0.8 vol. %.
 4. The method according to claim 2,wherein the oxygen content of the wood elements fed to the acetylationreactor is less than 0.8 vol. %.
 5. The method according to claim 1,comprising removing oxygen from the wood elements before the acetylationreaction, preferably by applying vacuum level of 100 mbar or less to thewood elements.
 6. The method according to claim 1, wherein theacetylation medium is supplied to the reactor in both gaseous and liquidform.
 7. The method according to claim 6, wherein the liquid iscontacted with the wood elements, preferably by spraying.
 8. The methodaccording to claim 5, wherein the wood elements after the removal ofoxygen are brought under the pressure of at least 1.5 barg before theacetylation.
 9. The method according to claim 1, wherein the acetylationmedium comprises acetic acid, acetic anhydride or a mixture thereof. 10.The method according to claim 1, wherein the weight ratio of theacetylation medium in the gaseous form to the liquid form supplied tothe acetylation reactor is in the range 0.5/1 to 4/1.
 11. The methodaccording to claim 1, wherein the wood elements have a size of a heightand width from 0.1 cm to 3 cm, and a length of 1 to 7 cm.
 12. The methodaccording to claim 1, wherein the acetylation is carried out at atemperature in the range 160-190° C.
 13. The method according to claim1, wherein the wood is acetylated to an acetyl content of at least 17wt. % measured by HPLC.
 14. The method according to claim 1, whereinsubstantially all of the acetylation medium is supplied to the woodelements after they are at a pressure of at least 1.5 barg.
 15. Themethod according to claim 1, wherein the acetylation reaction isconducted substantially in the absence of catalyst.
 16. The methodaccording to claim 1s, wherein the residence time is under 1 hour. 17.The method according to claim 1, wherein the acetylation reactorcomprises acetylation reaction zone (A) and wherein the wood elementshaving a temperature within +/−15° C. of the average temperature of theacetylation reaction are introduced into the acetylation reaction zone(A) having a pressure of at least 1.5 barg and a residence time of under1 hour.
 18. The method according to claim 17, wherein the wood elementsare saturated in the acetylation medium before being introduced into theacetylation reaction zone (A).
 19. The method according to claim 17,wherein the majority of the acetylation medium added to the reactor isadded upstream of acetylation reaction zone (A).
 20. The methodaccording to any one of claim 17, wherein the average residence time inacetylation reaction zone (A) is greater than 65%, preferably greaterthan 75%, more preferably greater than 80%, most preferably greater than85% of the residence time of the acetylation reactor.